1. Field of the Invention
This invention relates generally to a method and apparatus for measuring Raman spectra and physical properties of polymers, and more particularly, to a method and apparatus for measuring the properties of polymers in-situ.
2. Description of the Related Art
By measuring the physical properties of polymers during the production process, the quality and characteristics of the final product may be more easily monitored and adjusted. The physical properties that may be important to production include crystallinity, temperature, applied stress and orientational state of the polymer. The existing methods for measuring the physical properties of polymers include inherently destructive and non-destructive methods. The destructive methods employ devices that are placed in physical contact with the sample to measure physical properties thereof. For example, the temperature of a polymer sample may be measured by placing a thermocouple in contact with the polymer sample. Methods that use physical contact between the measuring device and a fragile sample involve a high risk of damage to the sample and perturbation of sample properties. The non-destructive methods are based on scattering or absorption of light or other electromagnetic radiation by the polymer sample without placing a measuring device in contact with the sample. The absorption and scattering methods include nuclear magnetic resonance, infrared and optical spectroscopy, birefringence measurements and X-ray diffraction. Unfortunately, the non-destructive methods often employ devices that are not well-suited to the environments encountered in a facility for manufacturing polymers.
FIG. 1 illustrates a device 10 for facilitating the in-situ measurement of Raman or vibrational spectra of a sample 12 in a manufacturing context. The device 10 consists of sensitive apparatus 13 that are preferably located in a protected environment and a remote probe 14 that may be located in the manufacturing environment. The sensitive apparatus 13 include a source 16 for providing the polarized illumination beam 18 that excites Raman scattering. The illumination beam 18 passes through a first optical train 20 that focuses the beam 18 onto a delivery optical fiber 22. The delivery optical fiber 22 is polarization preserving and either a single-modal or a multi-modal fiber. For example, a 10 .mu.m fiber has been used with an Argon ion laser source 16 that produces light with a wavelength of about 514.5 nm. The delivery optical fiber 22 carries the illumination beam 18 from the source 16 to the remote probe 14. A second optical train 24 creates a collimated beam 25 from divergent light leaving the delivery fiber 22. The collimated beam 25 is directed towards a beam splitter 26. The beam splitter 26 reflects a portion of the collimated beam 25 to produce a sample illumination beam 28. The sample illumination beam 28 passes through a third optical train 30 for focusing the beam 28 onto the sample 12. A portion of the light 32 scattered by the sample 12 re-enters the optical train 30 and forms a collimated return beam 33 that intercepts the beam splitter 26. A portion of the return beam 33 is transmitted through the beam splitter 26 and intercepts a removable mirror 34. The return light 35 reflected by the removable mirror 34 intercepts a fourth optical train 36. The optical train 36 focuses the reflected return light 35 onto a first end of a collection optical fiber 38. The collection fiber 38 is a multi-modal fiber, 50 .mu.m or larger that can carry a range of Raman frequencies. When the mirror 34 is removed from the path of the return beam 33, the return beam 33 follows a straight path 40 that intercepts a lens system 41 for focusing light on an optical input of a closed circuit television (CCTV) camera 42. When the mirror 34 is removed, the CCTV camera 42 may be used for white light imagining of the sample 12. The light leaving a second end of the collection fiber 38 is collimated by a fifth optical train 44 before entering a spectrometer 46. A charge coupled device (CCD) 47 changes the collected light into electrical signals that may be further analyzed. The spectrometer 46, e.g., a SPEX 1000M single monochromator, and the CCD 47, e.g., a Wright Instruments air cooled CCD with 300.times.1200 pixels, are the sensitive devices 13 that are located in a protected environment.
For materials such as polymers, background light can overwhelm the weak light produced by Raman scattering. The prior art device 10 employs a variety of features to increase the Raman scattering light to background light ratio. First, the source 16 is typically a monochromatic source such as a laser. Second, filters 48 and 50 reduce effects due to the Raman activity of the delivery and collection fibers 22 and 38.
If the illumination beam 18 has a wavelength of about 514.5 nm, the spectrum of the collimated beam 18 is mainly broadened by the Strokes Raman shift of fused silica. This broadening is substantially reduced by placing the filter 48 between the output of the delivery fiber 22 and the beam splitter 26. One construction for the filter 48 is a combination of a bandpass filter having an optical density of about 3 and a longpass filter having an edge at about 99 cm.sup.-1. The return light 33 includes components from both Raman and Rayleigh scattering.
The light from Rayleigh scattering may excite enough Raman activity in the collection fiber 38 to overwhelm the weaker light from Raman scattering by the sample 12. To reduce the Raman activity of the collection fiber 38, a notch filter 50 is placed between the mirror 34 and the collection fiber 38. The notch filter attenuates the source and Rayleigh light.
The two filters 48 and 50 act in combination to eliminate substantially all light from the collection fiber 38 except inelastically scattered light, i.e. Raman scattered light. The result is a Raman spectrum that is substantially independent of the power of the source 16 and primarily limited by electrical noise in the CCD 47.
The sensitivity of the remote probe 14 may be further improved by carefully coupling the optical components. First, the beams 18 and 35 are focused to a spot on the ends of the delivery and collection of fibers 22 and 38, respectively. The spot size is about equal to the diameter of the respective fibers 22 and 38 on which the beams 18 and 35 are incident. The optical trains 20 and 36 control the spot size for the beams 18 and 35, respectively. Second, the Raman excitation of the sample 12 is enhanced by adjusting the optical train 30 to form a focused spot 52 on the sample 12. A x80 Olympus MS Plan ultra-long working distance (4.7 mm) objective can produce a 0.5 .mu.m focused spot 52 on the sample 12. Third, the overall light throughput is improved by increasing the reflectivity of the beam splitter 26 to the collimated beam 25 and by increasing the transparency of the beam splitter 26 to the return beam 33. A holographic beam splitter may be highly reflective to incident light of a preselected wavelength and polarization and he highly transparent to other light. For example, a holographic beam splitter made by Kaiser Optical Systems Inc. of Ann Arbor, Mich. can reflect about 90% of properly polarized light at a wavelength of about 514.5 nm and transmits about 90% of the light at other wavelengths, characteristic of Raman scattering. To increase the reflectivity of the holographic beam splitter 26 an adjustable birefringent device 54 rotates the polarization so that the collimated beam 25 is polarized for optimal reflection. The polarization state selected by the birefringent device 54 is maintained in the polarization preserving delivery fiber 22. Increasing the light throughput through a combination of the above-mentioned devices can be important in applications to weak Raman scatters.
The device 10 of FIG. 1 has several drawbacks in on-line or in-situ applications. First, the device 10 is difficult to externally align, because filter 50 reduces the visibility of the laser light to about 1 part in 10.sup.6 of the intensity of the focused spot 52 by removing light not coming from Raman scattering. The low visibility of the filtered light beams makes alignment of the fifth optical train 44 with respect to the spectrometer 46 and the charge coupled device 47 difficult. Second, the internal alignment of the optical elements of the device 10 is difficult because of the visibility of the filtered beams 33, 40, and 35 and the fixed attachment of the optical assemblies 24 and 36. Third, the device 10 does not measure polarization dependence of the Raman component of the return light 35. Fourth, the device 10 is not made with off-the-shelf components and thus is rather expensive. Fifth, the remote probe 14 is not simply modified to allow changes to the operating specifications.
The present invention is directed to overcoming, or at least, reducing the effects of one or more of the problems set forth above.